Polarizing plates are widely used in displays such as liquid crystal displays and, particularly in recent years, in a variety of mobile devices such as smart phones and slate PCs. For use in mobile devices, polarizing plates are increasingly required to be thin and light. On the other hand, polarizing plates are also required to have high heat resistance.
However, conventional polarizing plates can cause a problem called “red discoloration,” in which light in the red region leaks from polarizing plates having undergone a heat resistance test (generally at 80 to 85° C. for 500 to 750 hours). Various methods have been proposed to suppress discoloration such as red discoloration.
Conventional methods for suppressing discoloration or decoloration of polarizers include, for example, a method of adding sulfate ions to polarizers (PTD 1); a method in which a process of immersing a film in an aqueous solution with a pH adjusted in a specific region is added to a polarizer producing process (PTDs 2 and 3); a method of forming a specific protective layer on a polarizer (PTD 4); a method of controlling the ratio between the contents of iodine and potassium in a polarizer (PTD 5); a method of improving the orientation of I3− adsorbed in a polarizer (PTD 6); and a method of adding zinc ions to a polarizer (PTDs 7 to 9).
In this regard, PTDs 10 to 12 disclose that the amount of variations in the retardation Rpva of polyvinyl alcohol is used as a measure of the uniformity of a polarizer. In these literatures, however, only Rpva is used for the definition and there is no description on the orientation of iodine (the retardation Ri of iodine).